Carbon dioxide is conventionally obtained as a gaseous by-product from the production of ammonia or hydrogen as well as from fermentation plants. It is known to convert the gaseous by-product into pure liquid carbon dioxide by distillation at recoveries exceeding 94% by weight.
Conventional distillation columns used for producing liquid carbon dioxide typically operate at a pressure of about 260 psia and a column condenser temperature of about -25.degree. F. The waste gas removed from the top of the column as an overhead stream, under these conditions, contains about 75% by volume of carbon dioxide. Accordingly, the amount of carbon dioxide lost as waste is about triple the amount of impurities in the feed. It therefore follows that carbon dioxide recovery decreases significantly as the concentration of carbon dioxide in the feed decreases.
The decreasing availability of carbon dioxide feedstocks of moderately high purity (e.g., in excess of 98% carbon dioxide, dry basis) has forced the development of techniques and plant modifications to effectively recover and liquefy streams containing substantially higher levels of non-condensible impurities. When substantial fractions of light contaminants are present in the feed stream to a carbon dioxide liquefaction plant, the quantity of carbon dioxide that can be effectively condensed and purified declines due to the dewpoint suppression/reduction of the feed/vent stream. FIGS. 1 and 2 of U.S. Pat. No. 4,639,257 (the '257 patent) graphically depict this dewpoint suppression effect, in which, if a gas mixture contains less than the equilibrium concentration of carbon dioxide at the freezing temperature of the mixture concerned, then the carbon dioxide cannot be separated by cooling and partial condensation or by cooling and distillation, since the carbon dioxide will freeze before any liquid is formed. Simply reducing the primary condensation temperature of the refrigerant (such as ammonia) in order to increase the carbon dioxide recovery in such a situation often leads to undesirable equipment complications and thermodynamic performance inefficiencies. In conjunction with such difficulties, modifying the carbon dioxide refrigeration device still does not address the lost work contained within the high pressure/residual carbon dioxide vent stream.
Numerous attempts have been made to overcome the above-described problem. The first and most direct means to reduce the carbon dioxide lost in the overhead vent stream of a carbon dioxide plant is simply to reduce the temperature at which the vent stream is condensed. As the vent temperature is reduced, the fraction of carbon dioxide condensed and recovered is increased. In order to generate lower vent condensation temperatures, refrigerants such as carbon dioxide or ammonia at sub-ambient pressures must be used to absorb the cold condensing duty. As a natural consequence of the thermodynamic limitations of the saturation temperature of ammonia, however, increasing the column pressure level is often the only available process variable that can be maximized for power maximization. But, increased column pressure is accompanied by a substantial increase in the pressure energy losses associated with the process vent. Notwithstanding their drawbacks, however, these options have been commercially utilized and are essentially industry standards for increasing carbon dioxide recovery from existing plants.
More recent processes have been proposed, which increase carbon dioxide recovery from vent streams by utilizing membrane and/or adsorption units. In the membrane arrangements, the carbon dioxide vent stream is subjected to a membrane that preferentially diffuses carbon dioxide. The permeate stream/carbon dioxide enriched stream is then reintroduced into the feed compression train in which the recycled carbon dioxide is condensed and recovered. Such hybrid membrane processes have been disclosed in the '257 patent as well as in U.S. Pat. Nos. 4,602,477 and 4,936,887. Analogously, adsorption systems have been proposed with similar objectives. In these arrangements, carbon dioxide preferentially adsorbs onto an adsorbent. The adsorbent vessel is then depressurized and/or the carbon dioxide enriched desorbing stream is extracted and reintroduced into the feed compression and condensation train. The waste stream from a distillation column is processed in a pressure swing adsorption apparatus to produce a highly concentrated carbon dioxide stream that is recycled to the carbon dioxide feed. U.S. Pat. No. 4,952,223 shows an example of a pressure swing adsorption (PSA) vent processing apparatus, in which pure liquid carbon dioxide is produced from low concentration carbon dioxide feeds, particularly feeds having a concentration of carbon dioxide of from about 35% to about 98% by volume.
As indicated, there are a number of hybrid processes that increase vent carbon dioxide recovery via diffusion (membranes) and/or adsorption (PSA/VPSA). In general, these processes are substantially different from the present invention in both operation and requisite equipment. A comparison of the performance of these hybrid processes to the present invention indicates that these arrangements have several associated disadvantages. The performance of membranes and pressure swing adsorption units for increasing vent carbon dioxide content is inferior to that achievable via partial condensation and/or distillation, as in the present invention. More importantly, membranes and pressure swing adsorption units do not mitigate the pressure energy losses of the residual vent stream. In addition, both membranes and pressure swing adsorption units sacrifice vent pressure energy. In effect, both processes substantially reduce the pressure of the enriched, recycle carbon dioxide stream (raffinate/desorbate). Further, since the recycle stream will be of a lower pressure, both hybrid processes have to incorporate either an additional recycle compressor or provide for an incremental feed compressor size increase.
Another technique proposed for reducing vent losses does not focus not upon recovering additional carbon dioxide. In contrast, this process recovers the pressure energy contained within the carbon dioxide vent stream. Most carbon dioxide liquefaction/distillation arrangements operate at substantially super-atmospheric pressures (e.g., in excess of about 20 atm). As a consequence, any non-condensibles and uncondensed carbon dioxide will naturally exit the distillation/condensation process at such a pressure. Typically, no attempt or provision is made to recover the contained vent pressure energy. However, as contaminant levels increase, so does the flowrate of the vent stream and the lost pressure energy. The most direct means to recover this pressure energy is by the use of an auxiliary turbo-expansion. In effect, the vent stream is warmed and expanded with the concomitant recovery of the shaft work of expansion. This option avoids the inefficient throttling of the vent stream and can save substantial amounts of power. U.S. Pat. No. 4,977,745 (the '745 patent) discloses such an arrangement.
In summary, the past attempts to reduce carbon dioxide vent losses have been primarily focused upon increasing either (i) carbon dioxide recovery or (ii) vent pressure energy recovery.